login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A039787 Primes p such that p-1 is squarefree. 15
2, 3, 7, 11, 23, 31, 43, 47, 59, 67, 71, 79, 83, 103, 107, 131, 139, 167, 179, 191, 211, 223, 227, 239, 263, 283, 311, 331, 347, 359, 367, 383, 419, 431, 439, 443, 463, 467, 479, 499, 503, 547, 563, 571, 587, 599, 607, 619, 643, 647, 659, 683, 691, 719, 743 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

An equivalent definition: numbers n such that phi(n) is equal to the squarefree kernel of n-1.

Minimal value of first differences (between odd terms) is 4. Primes p such that both p and p + 4 are terms are: 3, 7, 43, 67, 79, 103, 223, 439, 463, 499, 643, 823, ... - Zak Seidov, Apr 16 2013

LINKS

Zak Seidov, Table of n, a(n) for n = 1..1000

EXAMPLE

phi(43)=42, 42=2^1*3^1*7^1, 2*3*7=42.

p=223 is here because p-1=222=2*3*37

MAPLE

isA039787 := proc(n)

    if isprime(n) then

        numtheory[issqrfree](n-1) ;

    else

        false;

    end if;

end proc:

for n from 2 to 100 do

    if isA039787(n) then

        printf("%d, ", n) ;

    end if;

end do: # R. J. Mathar, Apr 17 2013

MATHEMATICA

Select[Prime[Range[132]], SquareFreeQ[#-1]&](* Zak Seidov, Aug 22 2012 *)

PROG

(MAGMA) [p: p in PrimesUpTo(780) | IsSquarefree(p-1)];  // Bruno Berselli, Mar 03 2011

(PARI) is(n)=isprime(n)&&issquarefree(n-1) \\ Charles R Greathouse IV, Jul 02 2013

CROSSREFS

Cf. A000010, A007947, A049092 (complement).

Sequence in context: A165318 A108184 A049091 * A226937 A227199 A129940

Adjacent sequences:  A039784 A039785 A039786 * A039788 A039789 A039790

KEYWORD

nonn

AUTHOR

Olivier Gérard

EXTENSIONS

More terms from Labos Elemer

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified October 23 05:48 EDT 2014. Contains 248411 sequences.